The cleaning of soil fractions contaminated with inter alia chlorinated products such as for instance tetrachloroethene and trichloroethene is no simple matter. In the past this cleaning was mainly carried out ex-situ, wherein water was pumped through the contaminated soil fraction to dissolve the chlorinated products, whereafter the water with the chlorine products dissolved therein was cleaned above ground. It will be apparent that such a method is not only time-consuming but also costly.
Another method, wherein the cleaning of the soil fraction takes place in-situ, comprises a two-step reaction in which anaerobic and aerobic operations are performed alternately. During the first step of this two-step reaction, possibly present tetra- and trichloroethene is converted anaerobically into cis-1,2-dichloroethene, while in the second step this latter is converted aerobically into carbonic acid, water and chloride. A drawback of this method is that it must be carried out in two steps, wherein different and incompatible conditions--i.e. anaerobic and aerobic--must be brought about for the growth of the micro-organisms involved in this process. There is moreover the danger that during the first step a significant quantity of carcinogenic vinyl chloride can be formed.
As an example of a publication in which the above stated two-step reaction is described can be mentioned the European patent with publication number EP-B-0 569 199. In this publication is described an in-situ method for the biological decomposition of halogenated organic compounds in aqueous soil fractions, wherein under the influence of anaerobic conditions an electron donor is supplied to the soil fraction in order to stimulate the dehalogenating bacteria acting under anaerobic conditions and wherein the biological formation of lower halogenated compounds is limited by maintaining a sulphate-reducing environment in the fraction with biological activity. When the greater part of the lower halogenated compounds has been removed, the anaerobic conditions are transformed into aerobic conditions by adding oxygen so that the bacteria acting under aerobic conditions are activated. The biological decomposition of halogenated organic compounds is continued using these aerobic bacteria so as to ultimately form non-hazardous organic compounds and water.